Schistosome infections are studied quantitatively in man and in experimental animals. Studies in progress are in the follwoing areas: fibrosis: Mouse strains develop markedly different degrees of hepatic fibrosis following infection with S. mansoni. Neither hepatic fibrosis nor other histologic parameters correlated with H-2 haplotype of the strains tested, including a limited number of congenic strains. In a single cross between strains showing high and low fibrosis, fibrosis, was low. Backcross experiments are in progress. Hepatic collagen content correlated directly with collagen synthesis in vitro. Extracts of hepatic granulomas from schistosome-infected mice stimulate fibroblast proliferation in vitro, but extracts from animals enroute to high or low fibrosis stimulate the fibroblasts equally. Resistance to infection: Using substrains of S. mansoni developed by Dr. Richards, we are investigating the resistance of mice to second infections. Two strains derived from the same patient, PRT-3 and PRC-3, induced markedly different degrees of resistance to reinfection after a bisexual first infection. PRT-3 induced excellent resistance to all challenge strains and PRC-3 induced poor resistance to all strains tested.